The inner ear is the organ that detects both sound pressure and linear and angular acceleration, and is vital for the senses of hearing and balance. The sensory regions of the inner ear contain the mechanosensory hair cells and their associated nonsensory supporting cells. Both the three-dimensional structure of the inner ear and the pattern and morphology of the cells that comprise the sensory region are crucial for precise function of the inner ear, and their disruption during development can lead to congenital balance and hearing deficits. Furthermore, during adult life, damage of the hair cells, which are not regenerated in mammals, can lead to late-onset hearing loss. The molecular mechanisms by which the three-dimensional structure of the inner ear is formed and by which the hair cells and their associated supporting cells are patterned are unclear, and our long-term goal is to elucidate these molecular mechanisms as a means for uncovering new strategies for the treatment of vestibular and hearing deficits. Receptor tyrosine kinase (RTK) pathways are essential for the cell-cell signaling events required for a number of developmental processes and their absence or misregulation are associated with congenital disease and cancer. We propose the study of the Sprouty (Spry) gene family, a family of modulators of RTK signaling, in mouse inner ear development. We have recently shown that Spry2 is a mouse deafness locus, and we propose to expand our understanding of the role of Spry genes in inner ear development. We have found that Spry1-/-; Spry2-/- mutants have perturbations in the three-dimensional structure of the inner ear epithelium, and we propose experiments to determine the cause of this morphogenetic defect. We have also found that Spry2 mutant mice develop extra hair and supporting cells during embryogenesis, and that SPRY2 may normally antagonize Fibroblast Growth Factor Receptor 1 (FGFR1) signaling in this process. We propose experiments to find the cellular mechanism by which these extra cells appear in the Spry2 mutant and to address the role of the FGFR1/SPRY2 signaling pathway in the determination of hair cell number. These studies should elucidate mechanisms for determining the structure of the inner ear epithelium and for controlling hair cell number which may aid in our understanding of vestibular abnormalities and suggest strategies for generating new hair cells to treat age-related hearing loss. Relevance: The inner ear is the organ that is crucial for both our senses of hearing and balance. We propose the study of a class of genes called Sprouty genes in formation of the inner ear to help in our understanding of balance disorders and to suggest new strategies for the treatment of age-related hearing loss. [unreadable] [unreadable] [unreadable]